As the minimum feature size of VLSI circuits drops to submicron regions, distortion due to optical diffraction can no longer be neglected. An established practice in microphotolithography to overcome distortions is to introduce deliberate distortions in the mask artwork. The corrections are usually based on experience gained by a process of trial and error. For the recently developed phase-shifting masks, experience is scarce, and intuitions are hard to develop. Therefore, a systematic technique to determine suboptimal, if not optimal, pre-distorted masks can facilitate conventional, chromeless, and phase-shifting mask design.
While phase-shifting masks are used to overcome the diffraction consequences of conventional (binary) masks, and hence provide better resolution than conventional masks, phase-shifting masks contain more information than a conventional transmission mask, and therefore require more complex patterning procedures. For example, a designer of phase-shifting masks should not only take into account the need to compensate for diffraction, but should also provide destructive interference at the wafer's surface for good resolution, therefore making use of the diffraction. Moreover, a designer should also build mask topography which projects a three-dimensional image that adapts to the expected wafer topography. Accordingly, there is a need for a method of designing and, therefore making masks which provides the aforedescribed characteristics.